Existing Group IVB metallocene catalysts for olefin polymerization generally consist of two components, precatalysts and cocatalysts. The precatalyst includes a Group IVB metallocene dichloride or a dialkyl complex composed of two aromatic five-membered ring systems that may be tethered by a bridging unit (ansa metallocene complexes). Aromatic ligands to Group IVB metal can be of the same or different type, including but not limited to: cyclopentadienyl, indenyl, fluorenyl, or their derivatives. The cocatalysts includes normally alumoxane (MAO), modified alumoxane (MMAO) or perfluoro (tetraphenyl)borate. The precatalyst does not show any activity toward olefin polymerization. The cocatalyst is essential to activate the precatalyst.
Al(MAO)/M(Group IVB metal) molar ratio of 1,000 in a Group IVB metallocene catalyst is typical but it can reach 20,000 in some cases (Kaminsky, W.; Arndt, M. Adv. Polym. Sci. 1997, 127, 144; Gladysz, J. A. Guest Editor, Special Issue for Frontiers in Metal-Catalyzed Polymerization. Chem. Rev. 2000, 100 (4)). The disadvantage of the current catalyst system is the incorporation of aluminum or fluorine into the polymer, which can cause serious problems when polyolefins are thermally decomposed.
To overcome these problems, a neutral Group IVB metallocene type of catalysts containing dicarbollide, [(C5Me5)(C2B9H11)]MMe (M=Ti, Zr, Hf), was developed. Fourteen electron, d0 bent-metallocene alkyl complexes of general type (C5R5)2M(R′)+ exhibit a rich insertion, olefin polymerization, and C—H activation chemistry which is highly sensitive to the structural and electronic properties of the (C5R5)2M fragment, the presence or absence of Lewis base, and counterion/cocatalyst properties. Replacement of a uni-negative C5R5− ligand of (C5R5)2M(R′)+ by the isolobal, di-negative dicarbollide ligand (C2B9H112−) reduces the overall charge by one unit but leaves the gross structural and metal frontier orbital properties unchanged.
The resulting neutral mixed sandwich complexes [(C5Me5)(C2B9H11)]M(R) show a variety of ligand exchange, insertion (alkenes, alkynes etc) and ligand C—H activation reactions characteristics of electrophilic metal alkyls (Crowther, D. J.; Baenziger, N. C.; Jordan. R. F. J. Am. Chem. Soc. 1991, 113, 1455). It is noteworthy that this type of complexes can catalyze the polymerization of ethylene with a moderate activity in the absence of any cocatalysts. The activity was 7.2×104 g of PE/(mol of Zr atom h)(Crowther, D. J.; Baenziger, N. C.; Jordan. R. F. J. Am. Chem. Soc. 1991, 113, 1455). The activity would be increased in the presence of R3Al (R=alykl).
[(C5Me5)(C2B9H11)]MMe are not thermally stable and can be converted into {[(C5Me5)(C2B9H11)]M}2CH2 upon heating. The latter is also an active catalyst for ethylene polymerization with a similar activity to its parent complex (Crowther, D. J.; Baenziger, N. C.; Jordan. R. F. J. Am. Chem. Soc. 1991, 113, 1455; Karol, F. J.; Kao, S.-C.; Brady III, R. C. U.S. Pat. No. 5,162,466), which is a significant progress in the field of Ziegler-Natta catalysis (Kaminsky, W.; Arndt, M. Adv. Polym. Sci 1997, 127, 144). This process is of great scientific and technological interests since it can avoid using expense MAO, thus eliminating possible contaminate of alumina in the polymeric materials. However, these catalysts cannot be used in industries due to low activities.